The present invention relates to a planetary gear device in which planetary gears are received in receiving portions formed in a carrier such that planetary gears can rotate about their own axes.
In general, a planetary gear device includes an inner gear, a sun gear and a carrier whose axes are aligned with a rotation axis. A plurality of retaining protrusions are formed on the carrier. The respective retaining protrusions extend in parallel with the rotation axis of the planetary gear device and are arranged on a circumference about the rotation axis in such a manner as to be away from each other. A receiving portion is formed between retaining protrusions which are adjacent to each other in the circumferential direction. A planetary gear is received in each receiving portion such that the planetary gear can rotate about its own axis. Each planetary gear is in meshing engagement with the inner gear and the sun gear. (see Japanese Patent Application Laid-Open Nos. H09-112657 and H09-144844)
In the conventional planetary gear device, each retaining protrusion is held in a cantilever fashion. For this reason, the retaining protrusions are low in strength and their distal end portions are readily deformed in a circumferential direction and in a radial direction. Accordingly, when force acts on the planetary gears in a circumferential direction and/or in a radial direction about the rotation axis at the time of differential rotation of the planetary gear device, the respective retaining protrusions are deformed and the respective receiving portions are readily deformed in accordance with deformation of the respective retaining protrusions. As a result, the planetary gears received in the receiving portions become unable to rotate smoothly. This results in a problem that the smooth rotation of the entire planetary gear device is prohibited.